vendor asynccontextmanager outside of greenlet dependency

Fixed installation issue where the ``sqlalchemy.dialects.mysql`` module
would not be importable if "greenlet" were not installed.

This was actually breaking the sphinx build for when greenlet
were not installed.

Fixes: #7204
Change-Id: Ia351c124a2f1ca44bafe20a97267ce20cb55808f

1 files changed, 7 insertions, 61 deletions

diff --git a/lib/sqlalchemy/util/_concurrency_py3k.py b/lib/sqlalchemy/util/_concurrency_py3k.py
index 7a4a2c361..55fe87c6a 100644
--- a/lib/sqlalchemy/util/_concurrency_py3k.py
+++ b/lib/sqlalchemy/util/_concurrency_py3k.py
@@ -1,5 +1,11 @@
+# util/_concurrency_py3k.py
+# Copyright (C) 2005-2021 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+
 import asyncio
-from functools import wraps
 import sys
 from typing import Any
 from typing import Callable
@@ -194,63 +200,3 @@ def get_event_loop():
             return asyncio.get_event_loop_policy().get_event_loop()
     else:
         return asyncio.get_event_loop()
-
-
-# vendored from py3.7
-
-
-class _AsyncGeneratorContextManager:
-    """Helper for @asynccontextmanager."""
-
-    def __init__(self, func, args, kwds):
-        self.gen = func(*args, **kwds)
-        self.func, self.args, self.kwds = func, args, kwds
-        doc = getattr(func, "__doc__", None)
-        if doc is None:
-            doc = type(self).__doc__
-        self.__doc__ = doc
-
-    async def __aenter__(self):
-        try:
-            return await self.gen.__anext__()
-        except StopAsyncIteration:
-            raise RuntimeError("generator didn't yield") from None
-
-    async def __aexit__(self, typ, value, traceback):
-        if typ is None:
-            try:
-                await self.gen.__anext__()
-            except StopAsyncIteration:
-                return
-            else:
-                raise RuntimeError("generator didn't stop")
-        else:
-            if value is None:
-                value = typ()
-            # See _GeneratorContextManager.__exit__ for comments on subtleties
-            # in this implementation
-            try:
-                await self.gen.athrow(typ, value, traceback)
-                raise RuntimeError("generator didn't stop after athrow()")
-            except StopAsyncIteration as exc:
-                return exc is not value
-            except RuntimeError as exc:
-                if exc is value:
-                    return False
-                if isinstance(value, (StopIteration, StopAsyncIteration)):
-                    if exc.__cause__ is value:
-                        return False
-                raise
-            except BaseException as exc:
-                if exc is not value:
-                    raise
-
-
-# using the vendored version in all cases at the moment to establish
-# full test coverage
-def asynccontextmanager(func):
-    @wraps(func)
-    def helper(*args, **kwds):
-        return _AsyncGeneratorContextManager(func, args, kwds)
-
-    return helper